This invention relates to a method for producing a dispersion of substantially immiscible liquids. More specifically, this invention relates to the liquid-liquid extraction of uranium from phosphoric acid produced by the acidulation of phosphate rock.
There are many chemical processes which involve the contacting of substantially immiscible liquids to form a dispersion. A chemical process of this type is liquid-liquid extraction. For example, in the recovery of uranium from wet-process phosphoric acid, the wet-process acid is contacted with an organic extractant to extract uranium from the acid. The pregnant extractant is then typically contacted with an aqueous stripping solution to strip uranium from the extractant.
There have been many proposals in the prior art of methods for contacting two substantially immiscible liquids such as in the liquid-liquid extraction of metal values. In the liquid-liquid extraction art, the organic extractant and the aqueous solution are commonly contacted in a mixer-settler. A mixer-settler typically includes a mixing chamber and a settling chamber in which liquid in the mixing chamber overflows into an upstream end of the settling chamber. The organic extractant and the aqueous solution are normally mixed in the mixing chamber and then overflow into the settling chamber where the phases are allowed to separate.
While mixer-settlers have found wide acceptance in the liquid-liquid extraction art, a mixer-settler is not ideally suited for all applications. This is particularly true in applications in which there is a tendency to form stable emulsions. One area in which the formation of stable emulsions can present a problem is in the recovery of uranium from wet-process phosphoric acid.
A large portion of commercial phosphate production is converted first to a relatively dilute phosphoric acid by the so-called "wet process" (as distinguished from the furnace process which produces elemental phosphorus by direct reduction of the ore). The producer first manufactures sulphuric acid, then uses it to digest the rock. The chemical reaction forms phosphoric acid and calcium sulphate. The latter is filtered out leaving an impure acid stream typically containing about 30% P.sub.2 O.sub.5. The 30% acid is generally evaporated to about 54% "merchant acid". The higher the acid concentration, the harder it is to extract the uranium, so the 30% stage is where the uranium extraction must take place.
All phosphate rock contains measurable amounts of organic material such as humic acids. For example, Florida phosphate rock contains as much as 0.1% and more organic material. When phosphate rock containing solid organic material is acidulated with sulphuric acid, the aqueous phase takes up the solid organic material which is of such small particle size that much of it passes through the gypsum filter.
The organic material, or black particulate, causes emulsions during uranium extraction. In particular, these emulsions will collect at the organic-aqueous interface in any solvent extraction process that utilizes an organic extractant. The volume of emulsion generated is often such that flooding occurs and the equipment must be shut down and cleaned out to remove the emulsion.
While a number of processes have been developed for removing a substantial portion of the solid organic material, residual solid organic material as well as residual inorganic solids are in the wet-process acid at the time of extraction. These and other impurities may cause an unacceptably large volume of stable emulsions during uranium extraction when employing many liquid-liquid contacting means such as mixer-settlers.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for producing a dispersion of substantially immiscible liquids.
A further object of the invention is to provide an improved method and apparatus for producing a dispersion of substantially immiscible liquids which minimizes the formation of stable emulsions.
Yet a further object of the invention is to provide an improved method and apparatus for the liquid-liquid extraction of metal values employing an organic extractant.
A still further object of the invention is to provide an improved method and apparatus for recovering uranium from wet-process phosphoric acid.